Radical Islamist groups last night gathered at a north London mosque for a conference entitled A Towering Day in History to mark September 11, amid threats of a counter-demonstration by the British National party.

The four-hour meeting, billed to hear speeches on the "positive outcomes of September 11" and the "US conspiracy against Islam and Muslims", had been advertised with posters showing the hijacked aircraft crashing into the twin towers.

The main organiser was Anjem Choudary, leader of the Al-Muhajiroun group. Other speakers included Sheikh Abu Hamza, the Egyptian cleric based at the mosque in Finsbury Park, Sheikh Omar Bakri Mohammed, a Syrian resident in Britain who advocates the introduction of sharia law, and Dr Muhammed Al-Mass'ari, a critic of the Saudi government.

"We felt the most fitting monument to those who died, both in New York, Afghanistan and in Jenin, is to work out how we can go forward," said Mr Choudary. "Mr Blair has not learned anything - if he is planning to bomb Iraq - about the presence of US and British troops on Muslim soil.

"We just wallow in the deaths and forget the causes. It's not helpful. The conference is not a celebration. We don't welcome the death of anyone. But [that day] was a milestone in relations between Muslims and non-Muslims."

Another organiser said: "We are looking at the aftermath and trying to find out what we can expect from America. As Muslims, we know there is a war against Islam."

The meeting was originally planned to be open to the pub lic at a larger venue. Conway Hall, in central London, could not be used because of security concerns, while Friends House, the Quakers' main hall in London, was closed to them because a previous meeting there by Islamist radicals had, according to a spokeswoman, "advocated violence".

Sheikh Abu Hamza was banned from preaching at the Finsbury Park mosque this year because of a sermon he gave. Zacarias Moussaoui, the so-called 20th hijacker in the September 11 attacks, and Richard Reid, the alleged shoe bomber, both attended the mosque.

The British National party said it was planning a rally near the mosque under the slogan: "Keep Britain out of foreign wars. Keep foreign wars out of Britain."